The present invention relates to a process for preparing phosphonitrilic chloride oligomers, and more particularly to an improved process for preparing tri- and tetra-phosphonitrilic chlorides in high yields.
It is well known that crystalline phosphonitrilic chloride oligomers, namely tri- and tetra-phosphonitrilic chlorides are the starting materials for preparing poly-phosphonitrilic chloride generally called inorganic rubber. The use of poly-phosphonitrilic chloride derivatives of various kinds is very wide and they are employed availably as plastics, rubbers, plasticizers and fertilizers. Particularly, poly-phosphonitrilic chloride derivatives in use as plastics are watched in point of the flame-resistance or nonflammability, since combustion of plastics becomes a social problem.
The phosphonitrilic chloride oligomers were found first in the year 1834 by Liebig as by-products in the reaction of ammonium chloride and phosphorus pentachloride which was conducted for the purpose of synthesizing phosphonitrilic amide NP(NH.sub.2).sub.2. After that, many researches as to the phosphonitrilic chloride oligomers have been made, and typical methods for synthesizing the oligomers hitherto known are as follows:
[A] Methods using phosphorus pentachloride
(a) PCl.sub.5 +NH.sub.4 Cl PA1 (b) PCl.sub.5 +NH.sub.3 PA1 (c) PCl.sub.5 +NH.sub.3 +HCl PA1 (d) PCl.sub.3 +Cl.sub.2 +NH.sub.4 Cl PA1 (e) PCl.sub.3 +Cl.sub.2 +NH.sub.3 PA1 (f) P+Cl.sub.2 +NH.sub.3 PA1 (g) P.sub.3 N.sub.3 +Cl.sub.2 PA1 (h) P.sub.4 N.sub.5 +Cl.sub.2
[B] Methods using phosphorus trichloride
[C] Method using white phosphorus
[D] Methods using phosphorus nitride
Among these typical synthetic methods [A] to [D], the methods [D]-(g) and [D]-(h) are not suited for the synthesis on an industrial scale, because of being in danger of explosion.
Method on which the most researches have been made up to now is the method [A]-(a) using phosphorus pentachloride and ammonium chloride. The method [A]-(a) is classified into two large methods. One of them is a method is which phosphorus pentachloride and 1.0 to 2.0 moles of ammonium chloride per mole of phosphorus pentachloride are first added to a reaction vessel, and then reacted in an organic solvent for 15 to 30 hours at a boiling point of the solvent under reflux. The yield of the phosphonitrilic chloride oligomers is low and at most 30% to 40% by weight. Another is a method in which the reaction is carried out by adding dropwise a solution of phosphorus pentachloride in an organic solvent to a dispersion of ammonium chloride in the same organic solvent. This method is the improvement of the former to reduce the production of higher cyclic phosphonitrilic chlorides (NPCl.sub.2).sub.5-11 and liner compounds (NPCl.sub.2).sub.n.PCl.sub.5, which are inevitably by-produced at the time of preparing the phosphonitrilic chloride oligomers, particularly tri- and tetra-phosphonitrilic chlorides necessary for preparing polyphosphonitrilic chloride. The yield of the oligomers is raised to 50% to 60% by weight. However, it is necessary to previously dissolve phosphorus pentachloride in an organic solvent at a temperature of 80.degree. to 90.degree. C., and moreover it is necessary to maintain a pathway for adding dropwise the solution of phosphorus pentachloride at the above temperature. Therefore, this method is not also necessarily suitable for the industrial preparation.
The method [A]-(b) has the disadvantages that in addition to choking of a tube for introducing NH.sub.3 gas and difficulty of controlling the amount of flowing NH.sub.3 gas, the yield of the oligomers is only about 20% to about 30% by weight.
The method [A]-(c) is conducted by first supplying ammonia gas and hydrogen chloride gas into an organic solvent to produce ammonium chloride, and then adding phosphorus pentachloride into the solvent to react them so as to produce the oligomers. The object of this method is to make ammonium chloride particles as small as possible in order to raise the yield of the oligomers and to shorten the reaction time. For this purpose, the temperature for producing ammonium chloride in advance of the reaction of ammonium chloride and phosphorus pentachloride is desirable to be lower than 30.degree. C. to prevent the increase of the particle size of the produced ammonium chloride particles. Since the reaction of producing ammonium chloride is an exothermic reaction, a cooling apparatus is required. Therefore, there are required thermal procedures of two stages which are contrary to each other, at the first stage of which ammonia and hydrogen chloride are reacted to produce ammonium chloride with cooling and at the second stage of which the resulting ammonium chloride and phosphorus pentachloride are reacted with heating. This is industrially disadvantageous.
The method [B]-(d) which has been studied is a method where phosphorus trichloride is added to a reaction medium at once before starting the reaction. In such a method, it is difficult to smoothly start the reaction at a temperature of more than 100.degree. C., since the boiling point of phosphorus trichloride is low, i.e. about 75.degree. C.
The methods [B]-(e) and [C]-(f) have the disadvantage of choking of a tube for introducing NH.sub.3 gas.
It is also known to employ an anhydrous metal chloride such as aluminum chloride, zinc chloride, magnesium chloride or titanium chloride as a catalyst in order to shorten the reaction time. However, these metal chloride catalysts are deliquescent or easy to hydrolyze and, therefore, are unhandy. Moreover, they have a little effect of raising the yields of the phosphonitrilic chloride oligomers and only have the effect of shortening the reaction time.